As current processes and technologies used in connection with the production of semiconductor circuits scale further into deep submicron levels, the reduction in gate oxide thickness requires that the voltages around a circuit have to be reduced as well. Hence, in recent years, designing important analog building blocks for lower voltage performance has become a primary concern. Furthermore, careful design of these analog building blocks in a lower voltage environment leads to reduced power consumption, which is extremely important in hand-held and mobile devices. Designing with a lower supply voltage for RF building blocks is all the more important, since the RF front-end of mobile devices has always been the culprit in terms of supply voltage and power consumption.
FIG. 1 on the appended drawing illustrates an embodiment of a known RF front-end receiver.
In a manner known per se, the RF front-end receiver in FIG. 1 comprises a low noise amplifier LNA and a local oscillator driver LOD, which are connected to respective input ports of a mixer.
The low noise amplifier LNA receives an RF signal RF and supplies it to one mixer input port and the local oscillator driver LOD receives a local oscillator signal LO and supplies it to the other mixer input port. From the RF signal RF and the local oscillator signal LO, the mixer generates an intermediate frequency signal on its output terminals IF+, IF−.
The mixer is a variant of a standard Gilbert cell mixer and comprises a transconductance stage and a switching stage. By means of mixers of the type shown in FIG. 1, it is possible to use lower supply voltages.
The transconductance stage comprises two transistors M1 and M2. The gates of the transistors M1 and M2 form the mixer input port for the RF signal RF and are coupled to respective output terminals RF+, RF− of the low noise amplifier LNA. The sources of the transistors M1 and M2 are interconnected to ground, and the drains of the transistors M1 and M2 are coupled to respective interconnected sources of switching transistors M3, M4 and M5, M6, respectively, of the mixer switching stage.
In the switching stage of the mixer in FIG. 1, the gates of the transistors M3, M6 and M4, M5, respectively, are interconnected and form the mixer input port for the local oscillator signal, that is coupled to the output terminals of the local oscillator driver LOD.
The drains of the transistors M3, M5 and M4, M6, respectively, are interconnected and form the mixer output port IF+, IF− for the intermediate frequency signal.
The mixer in FIG. 1 will operate in a balanced manner as long as the input RF signal from the low noise amplifier LNA is differential. Since the mixer is not fully differential, it will not provide any common mode rejection. Common mode feedback can be employed at the IF output port, but nonetheless the mixer will have a lower common mode rejection ratio than a standard Gilbert cell mixer. In order to maintain reasonable common mode rejection in the RF front-end, it is important that the LNA that precedes this mixer is fully differential.